This invention relates to an optical pickup for recording and reproducing information relative to an optical element or an optical disk, and also relates to a method of producing the optical pickup.
It has heretofore been desired to achieve a compact design of an optical disk unit capable of recording and reproducing information using a laser beam, and attempts have been made to achieve a compact and lightweight design of an optical pickup by reducing the number of optical parts. The compact and lightweight design of the optical pickup contributes not only to a reduced overall size of the optical disk unit, but also to an enhanced performance such as a shortened access time. Recently, a hologram optical pickup has been proposed in order to provide a compact and lightweight design, and some of such proposals have been put into practical use.
A conventional hologram optical pickup will now be described with reference to FIGS. 35a to 38. FIG. 35a is a plan view of the conventional optical pickup, and FIG. 35b is a side-elevational view of the conventional optical pickup.
Reference is first made to an outward optical path from a semiconductor laser (light-emitting device) to an optical disk. In FIG. 35b, a laser beam, emitted horizontally from a semiconductor laser chip 2 mounted horizontally on a sensor base plate 1, is caused by a trapezoidal prism 4 (which is mounted on the sensor base plate 1 with its reflecting surface opposed to the semiconductor laser chip 2) to enter the interior of a transparent optical guide member 5 through an incident window 6 on a second surface 5b of the optical guide member 5, so that the laser beam is turned into diffused light 7 in the optical guide member 5. A hologram 8 is formed on a first surface 5a of the optical guide member 5, and the diffused light 7, going out of the optical guide member 5, passes through the hologram 8, and is turned into diffused light 9. The diffused light 9 is incident on an objective lens 10, and is converted into outward convergent light 13 which is then condensed into a spot 12 on an information recording layer 11a of the optical disk 11.
Next, reference is made to a return optical path from the optical disk to light-receiving sensors. Reflected light 14 from the information recording layer 11 of the optical disk passes through the objective lens 10, and is converted into return convergent light 15, and then is incident on the hologram 8.
As shown in FIG. 36, the hologram 8 is divided into two regions having an equal area by a division line (boundary) extending in the same direction as that of a track of the optical disk 11, the two regions having different patterns, respectively. The hologram 8 converts the return convergent light 15 into first diffraction light 16 and second diffraction light 17 diffracting in different directions at an angle of (2n+1) .pi./4, such as 45.degree. and 135.degree., relative to the polarization direction of the semiconductor laser chip 2.
A first return polarized light splitting portion 18 and a second return polarized light splitting portion 19 are formed on the second surface 5b of the optical guide member 5, and each of the two splitting portions 18 and 19 is formed of a return polarization separation film coated on the second surface 5b, and transmits a P polarization component of the diffraction light 16, 17 therethrough, but reflects an S polarization component thereof.
If the condition of polarization of the diffused light 7 incident on the hologram 8 is represented by linearly-polarized light 23 as indicated by an arrow in FIG. 35a, the P polarization component and S polarization component of each of the two diffraction light beams 16 and 17 become about a half relative to a respective one of the two return polarized light splitting portions 18 and 19, since the direction of diffraction of the two diffraction light beams 16 and 17 is set to (2n+1) .pi./4 relative to the polarization direction of the linearly-polarized light 23. Therefore, the light amount of each of first and second transmitted light beams 24 and 25, emerging respectively from the two return polarized light splitting portions 18 and 19, is about a half of the light amount of a respective one of the first and second diffraction light beams 16 and 17. The two transmitted light beams 24 and 25 are applied respectively to first and second light-receiving sensors 26 and 27 formed on the sensor base plate 1. First and second reflected light beams 28 and 29 (which are the remainder or the other halves of the two diffraction light beams 16 and 17, respectively), reflected respectively by the two return polarized light splitting portions 18 and 19, are further reflected respectively by first and second reflecting portions 30 and 31 on the first surface 5a to respectively form third and fourth reflected light beams 32 and 33 directed toward the second surface 5b. The third and fourth reflected light beams 32 and 33 pass respectively through first and second transmission windows 34 and 35 on the second surface 5b to form third and fourth transmitted light beams 36 and 37, respectively, which are applied to third and fourth light-receiving sensors 38 and 39, respectively. The focus of the diffraction light 16 is located between the return polarized light splitting portion 18 and the third light-receiving sensor 38 while the focus of the diffraction light 17 is located between the return polarized light splitting portion 19 and the fourth light-receiving sensor 39.
Next, an opto-magnetic signal detection principle will now be described in detail. In FIG. 37, reference numeral 23 denotes the direction of polarization of the linearly-polarized light 23 incident on the hologram 8 as described above. The hologram 8 does not exert any influence on the polarization plane, and therefore if any information is not stored on the information recording surface 11a of the optical disk 11 (that is, the information recording surface 11a is not magnetized), the two diffraction light beams 16 and 17, which are the reflected light of the spot 12, have the same polarization direction as that of the linearly-polarized light 23. With respect to the polarization direction of the two diffraction light beams 16 and 17 in this condition, the directions of diffraction of the two diffraction light beams 16 and 17 are set respectively to 45.degree. and 135.degree. with respect to the polarization direction of the linearly-polarized light 23 so that the diffraction light beams 16 and 17 can be incident respectively on the two return polarized light splitting portions 18 and 19 (which transmit substantially 100% of the P polarization component therethrough, and reflect substantially 100% of the S polarization component) with respective bearings of 45.degree. and 135.degree. as shown in FIG. 37. The direction of rotation of the linearly-polarized light 23, when reflected by a magnetized information pit in the optical disk 11, varies in the range of .+-..theta.k depending on the polarity and magnitude of the magnetization (Kerr effect).
Here, let's assume that linearly-polarized light 40 is obtained by rotating the linearly-polarized light 23 by an angle of .theta.k, and that linearly-polarized light 41 is obtained by rotating the linearly-polarized light 23 by an angle -.theta.k. Here, let's consider the case where the optical signal, modulated from the linearly-polarized light 40 to the linearly-polarized light 41 by the recording magnetic field of the optical disk 11, is incident on the polarized light splitting films of the two polarized light splitting portions 18 and 19.
When the polarization condition of the return convergent light 15 is rotated .theta.k from the condition of the linearly-polarized light 23, the polarization condition of the first diffraction light 16 is modulated into the linearly-polarized light 40 while the polarization condition of the second diffraction light 17 is modulated into the linearly-polarized light 41. When the polarization condition of the return convergent light 15 is rotated -.theta.k from the condition of the linearly-polarized light 23, the polarization condition of the first diffraction light 16 is modulated into the linearly-polarized light 41 while the polarization condition of the second diffraction light 17 is modulated into the linearly-polarized light 40. Therefore, the P polarization component of the second diffraction light 17 is equal to the S polarization component of the first diffraction light 16, and the S polarization component of the second diffraction light 17 is equal to the P polarization component of the first diffraction light 16. An RF reproducing signal is doubled in its signal component because of the differential (expressed by formula (1) described later) between the sum of the signal of the P polarization component of the first diffraction light 16 and the signal of the S polarization component of the second diffraction light 17 and the sum of the signal of the S polarization component of the first diffraction light 16 and the signal of the P polarization component of the second diffraction light 17, and noises in the components of the same phase are canceled, so that the signal with a high C/N ratio can be obtained.
Signals are inputted to and outputted from the sensor base plate 1, having the semiconductor laser chip 2 and the group of light-receiving sensors, through a lead frame 44.
Next, the configuration of the first, second, third and fourth light-receiving sensors 26, 27, 38 and 39, as well as a signal detection principle, will be described with reference to FIG. 38.
The second light-receiving sensor 27 and the fourth light-receiving sensor 39 are a three-division type, that is, the former is divided into three sections 27a, 27b and 27c while the latter is divided into three sections 39a, 39b and 39c. Here, outputs of the first and third light-receiving sensors 26 and 38 are represented by I26 and I38, respectively, outputs of the three sections 27a, 27b and 27c of the second light-receiving sensor 27 are represented by I27a, I27b and I27c, respectively, and outputs of the three sections 39a, 39b and 39c of the fourth light-receiving sensor 39 are represented by I39a, I39b and I39c, respectively.
The RF reproduction signal (R. F.) among the various signals will first be described. As described above, the RF reproduction signal is obtained by the differential between the sum signal representative of the sum of the P polarization component of the diffraction light 16 and the S polarization component of the diffraction light 17 and the sum signal representative of the sum of the S polarization component of the diffraction light 16 and the P polarization component of the diffraction light 17, and therefore R. F. is obtained from the following formula as will be appreciated from a circuit diagram of FIG. 38: EQU R. F.=[I26-(I27a+I27b+I27c)]-[I38-(I39a+I39b+I39c)]
Next, a focus error signal (F. E.) will be described. F. E. is obtained from the following formula as will be appreciated from the circuit diagram of FIG. 38: EQU F. E.=[(I27a+I27c)+I39b]-[(I39a+I39c)+I27b]
Here, let's assume that the spot 12 formed by the objective lens 10 is accurately in focus on the information recording layer 11a of the optical disk 11, and that in this focused condition, the configuration of irradiation of the laser beam on the second light-receiving sensor 27 is represented by 45a while the configuration of irradiation of the laser beam on the fourth light-receiving sensor 39 is represented by 46a. Then, the laser beam-irradiating intensity distribution and the positions of the light-receiving sensors are so adjusted that the following formula can be established: EQU F. E.=0
Next, when the distance between the optical disk 11 and the objective lens 10 decreased from that in the above-mentioned focal distance condition, the configuration of irradiation of the laser beam on the second light-receiving sensor 27 is represented by 45c while the configuration of irradiation of the laser beam on the fourth light-receiving sensor 39 is represented by 46c, and F. E. is varied as indicated in the following formula: EQU F. E.&gt;0
In contrast, when the distance between the optical disk 11 and the objective lens 10 increases from the above-mentioned focal distance condition, the configuration of irradiation of the laser beam on the second light-receiving sensor 27 is represented by 45b while the configuration of irradiation of the laser beam on the fourth light-receiving sensor 39 is represented by 46b, and F. E. is varied as indicated in the following formula: EQU F. E.&lt;0
This focus error detection method is known as a spot-size method.
Next, a tracking error signal (T. E.) will be described. The boundary between the two regions of the hologram 8, having the same area and the different patterns, is extended in the same direction as that of the track of the optical disk, and therefore track information, contained in the reflected light from the optical disk 11, is divided by the hologram 8 into two (right and left) track informations divided by the centerline of the spot 12 extending in the direction of the track. The two track informations are divided into the first and second diffraction light beams 16 and 17. The hologram 8 is so designed that the two regions for the first and second diffraction light beams 16 and 17 have the same diffraction efficiency. Therefore, T. E. is obtained from the following formula as will be appreciated from the circuit diagram of FIG. 38: EQU T. E.=[(I27a+I27b+I27c)+(I39a+I39b+I39c)]-(I26+I38)
When the spot 12 is applied to the centerline of the track, the two regions of the hologram 8 receive the return convergent light 15 in an equal amount, and therefore the two diffraction light beams 16 and 17 are equal in light amount to each other, and the sum signal representative of the sum of the signal of the first light-receiving sensor 26 and the signal of the third light-receiving sensor 38 (which are representative of the light amount of the first diffraction light 16) is equal to the sum signal representative of the sum of the signal of the second light-receiving sensor 27 and the signal of the fourth light-receiving sensor 39 (which are representative of the light amount of the second diffraction light 17). Therefore, T. E. is expressed by the following formula: EQU T. E.=0
When the spot 12 is displaced from the centerline of the track in a direction at 90.degree. with respect to the track, the two regions of the hologram 8 receive the return convergent light 15 in different amounts, respectively, and the sum signal representative of the sum of the signal of the first light-receiving sensor 26 and the signal of the third light-receiving sensor 38 (which are representative of the light amount of the first diffraction light 16) is not equal to the sum signal representative of the sum of the signal of the second light-receiving sensor 27 and the signal of the fourth light-receiving sensor 39 (which are representative of the light amount of the second diffraction light 17). Therefore, T. E. is expressed by either of the following formulas: EQU T. E.&gt;0 EQU T. E.&lt;0
This tracking error detection method is known as a push-pull method.
Thus, the hologram 8 is divided into the two regions of the same area (which have the respective different patterns) by the division line (boundary) extending in the same direction as that of the track of the optical disk 11, and the two regions for the two diffraction light beams 16 and 17 have the same diffraction efficiency. With this design, the tracking error signal can be obtained.
In the manufacture of an optical pickup as disclosed in Japanese Patent Unexamined Publication Nos. 5-258382 and 5-258386, optical guide elements having optical elements provided at boundary surfaces, are laminated or bonded together to form an assembly block and then inclined surfaces are formed. Thereafter, polarization films are formed, and then the assembly block is cut into a predetermined size to thereby provide an optical pickup element.
In the above conventional construction, however, the following problems are encountered, so that the C/N ratio of the RF signal is adversely affected:
Although a hologram is used for separating outward light and return light from each other, zero-order diffraction light is used in the outward path while first order light is used in the return path, and because of a limited angle of incidence on a polarized light splitting portion, the blazing pitch of the hologram is small, and the suppression of the first order diffraction light by blazing is difficult, and besides the first order diffraction efficiency of the S polarization component is lower than the first order diffraction efficiency of the P polarization component. Therefore, the improvement of the efficiency of use of the outward and return light beams (the zero-order diffraction efficiency of the outward light x the first order diffraction efficiency of the return light) is limited, and a loss of the light amount at the hologram is large.
Since the RF signal is also detected by light-receiving sensors of the divided type which detects a focus error, a loss of the light amount occurs at a dead zone of the division portion.
Since a beam splitter film having polarization selectivity is not used, an enhancing effect for increasing the apparent Kerr rotation angle is not obtained.
In addition, the following problems are encountered, so that the focus error signal and the tracking error signal are adversely affected:
The focus error signal is detected by the size of a P polarized-light spot on the light-receiving sensor and the size of a S polarized-light spot on the light-receiving sensor, and therefore when the light amount ratio of the P polarized-light to the S polarized-light of the diffraction light is varied by birefringence, the Kerr rotation and so on, the focus error signal is subjected to offset.
Since the tracking error signal is detected by a push-pull method, this detection is likely to be affected by the depth of pits and grooves in the optical disk.
In the case where the tracking error signal is detected by a 3-beam method using a diffraction grating, crosstalk between a main beam and side beams occurs since the size of the spot on the light-receiving sensor is large.
In the manufacture of the optical pickup as disclosed in Japanese Patent Unexamined Publication Nos. 5-258382 and 5-258386, the number of the optical pickup elements produced from one assembly block is determined by the length of the optical guide members, and besides, since the assembly block has the inclined surfaces, it is difficult to arrange the pickup elements of the same configuration in the direction of the width of the assembly block, which results in a low productivity.
U.S. Pat. No. 5,095,476 and Japanese Patent Unexamined Publication No. 62-117150 are other prior art publications.